1. Field of the Invention
The present invention relates to an improved continuously charged Czochralski method (i.e., a CCZ method) of manufacturing a silicon monocrystal and, more particularly, to a method of manufacturing a silicon monocrystal whose resistivity is controlled so as to fall within a predetermined range. Further, the present invention relates to a dopant feeding apparatus suitable for use in the manufacturing method.
2. Description of the Related Art
There exists a CCZ method in which a silicon monocrystal is pulled while silicon material is supplied to silicon melt whose volume reduces as a result of pulling of the silicon monocrystal. This method enables the pulling of a longer silicon monocrystal from one crucible, thereby improving manufacturing yield and reducing manufacturing costs of the silicon monocrystal.
In the ordinary CZ method, in order to control the resistivity of a silicon monocrystal to be pulled, the silicon monocrystal is pulled while dopant; e.g., boron, phosphorous, or antimony, is added to the silicon melt. Introduction of dopant into the silicon monocrystal is dependent on segregation. As the pulling of silicon monocrystal proceeds, the dopant concentration of the remaining silicon melt increases, and, as a result, the resistivity of the silicon monocrystal to be pulled gradually decreases.
At this time, the resistivity of the silicon monocrystal is determined by an exponential curve defined by an effective segregation coefficient (k.sub.eff) of the dopant. The term "effective segregation coefficient (k.sub.eff) of the dopant" is used herein to refer to a segregation coefficient when a silicon monocrystal is actually grown. For example, if the resistivity is measured at both longitudinal ends of the silicon monocrystal block, the distribution of resistivity of the area of silicon monocrystal block between its longitudinal ends can be determined from the exponential curve. Because of this, it is possible to confirm which portion of the silicon monocrystal block has a desired range of resistivity without the need of any further measurements.
However, according to the ordinary CZ Method, the resistivity of the silicon monocrystal to be pulled decreases exponentially for reasons of segregation, and therefore the resistivity of the latter half of the silicon monocrystal deviates from a desired range of resistivity, thereby resulting in decreased manufacturing yield.
To prevent the previously-described problem, an improved CCZ method has been developed. According to this method, the silicon monocrystal is pulled while its resistivity is controlled by charging silicon material to the silicon melt whose volume reduces as a result of pulling of the silicon monocrystal, so as to prevent the dopant concentration of the silicon melt from increasing. In this method, if the silicon material is charged so as to maintain the dopant concentration of the silicon melt constant, the resistivity of the silicon monocrystal to be pulled can be maintained constantly. However, it will be impossible to pull a too long silicon monocrystal without addition of the dopant. In some cases, a longer silicon monocrystal is pulled through addition of dopant as well as silicon material to the silicon melt.
In this case, there is a method of pulling a silicon monocrystal while charging to silicon melt a mixture of granular silicon material and dopant-containing granular silicon material that has the same shape distribution as the granular silicon material and is capable of being evenly mixed with the granular silicon material. The granular silicon material and the dopant-containing granular silicon material are mixed in a suitable ratio. The dopant-containing granular silicon material is special material, and therefore this method is impracticable.
There is still another method of pulling a silicon monocrystal while continuously charging dopant--which is different in shape distribution from the granular silicon material and does not evenly mix with the granular silicon material--to the silicon melt separately from the granular silicon material. In this method, since the amount of dopant to be continuously charged is small, a large amount of errors arise in the amount of dopant to be added. As a result, variations in the resistivity of the silicon monocrystal to be pulled become greater. For this reason, this method is also impracticable.
In the previously-described methods, there may be used a double crucible comprising an outer crucible and an inner crucible which communicate with each other through pores. Use of the double crucible is intended to control the resistivity of a silicon monocrystal to be pulled by continuously charging dopant to the silicon melt stored in the outer crucible, and by causing the thus-charged dopant to flow into the inner crucible through the pores. Since it is impossible to measure the resistivity of the silicon monocrystal while it is being pulled, it is impossible to control the amount of dopant to be charged to the silicon melt of the outer crucible on the basis of feedback on the resistivity of the silicon monocrystal. Therefore, the resistivity of a resultantly-obtained silicon monocrystal may deviate from a desired range.
In contrast to the ordinary CZ method, even if the resistivity is measured at both longitudinal ends of the silicon monocrystal block, the distribution of resistivity of the area of the silicon monocrystal between its longitudinal ends cannot be determined, because the dopant is continuously charged during the course of pulling of the silicon monocrystal. Because of this, without further measurement of resistivity it is impossible to confirm which portion of the silicon monocrystal block has a desired range of resistivity.